1. Field of the Invention
This invention relates to an electroless or chemical gold plating bath and more particularly, to an electroless gold plating bath suitable for forming a gold plating film on metal portions on printed circuit boards, ceramic IC packages, ITO substrates, IC cards, etc.
2. Prior Art
For utilizing the advantages of gold including its physical properties such as electric conduction and thermal bonding ability and its chemical properties such as oxidation resistance and chemical resistance, electroless gold plating has heretofore been applied to terminals and circuit surfaces of electronic industry parts including printed circuit boards, ceramic IC packages, ITO substrates, and IC cards.
Conventional well-known electroless gold plating baths are strongly alkaline baths containing cyanoaurates, caustic alkalis and boron hydrides. Because of strong alkalinity, however, these electroless gold plating baths undesirably attack the resist of printed circuit boards and aluminum nitride and glass material of IC packages. Inclusion of a cyanide ion gives rise to a problem against environmental protection.
Also known in the art are neutral baths containing sulfites and hydrazines. Gold plating films resulting from these baths look discolored to a reddish color tone to naked eyes because the films are made of small and angular crystals. This crystallography also prevents the plating films from lending themselves to surface cleaning and gold wire bonding. Since there customarily prevails the empirical knowledge that gold plating films having good function have an aesthetic lemon yellow color tone, those gold plating films of poor tone are rather unacceptable as commercial products, that is, of lower commercial value even if they are susceptible to wire bonding.
In addition, the neutral baths tend to incur plating spread in that a coating also deposits at non-intended places, causing stains or blur. This tendency is exacerbated especially in the plating of a metal portion on a ceramic base, resulting in a plating film extending from the metal portion to the surrounding ceramic base surface.